Prior to 2000, Recommended Dietary Allowances (RDAs) for vitamin C (ascorbic acid), as well as other water-soluble vitamins, were based on preventing deficiency with a margin of safety. We proposed that new RDAs for vitamins, with vitamin C as a model, should be based on concentration-dependent vitamin functions, especially molecular and clinical functions. We termed this overall concentration-function approach in situ kinetics, with both molecular and clinical goals. Principles of in situ kinetics were adopted and expanded by the National Academy of Sciences as part of revised recommendations for vitamin C intake released in 2000. Molecular goals of in situ kinetics are to determine vitamin-specific functions in relation to vitamin concentrations, using biochemical and molecular techniques. Vitamin C function is investigated in human cells such as fibroblasts and neutrophils. To learn how intracellular concentration is regulated, two pathways of vitamin C accumulation were characterized. In the first pathway, vitamin C is transported as such by two carriers that are sodium-dependent, saturable, energy dependent, and inhibited by laboratory-synthesized ascorbate analogs. The two human transporters SVCT1 and SVCT2 were cloned and characterized, and genomic characterization and studies of nucleotide polymorphisms in human are underway. Created mice deficient in the transporter SVCT2 did not survive the perinatal period and had very low or undetectable vitamin C concentrations in many but not all tissues, indicating that vitamin C as such is the dominant transported species. In the second pathway the extracellular oxidized form of vitamin C, dehydroascorbic acid, is accumulated as vitamin C within neutrophils by the process of ascorbate recycling. Oxidants from neutrophils oxidize extracellular vitamin C to dehydroascorbic acid. Dehydroascorbic acid is transported by facilitative glucose transporters GLUT I, III, and IV, and immediately reduced intracellularly to vitamin C by glutaredoxin (thioltransferase). Glutaredoxin from neutrophils was isolated, identified as the reducing activity, cloned, and characterized. Our studies show that vitamin C recycling occurs in neutrophils when activated by bacteria, and only in neutrophils and not in bacteria. Studies are ongoing to characterize potential roles of vitamin C in neutrophils, including oxidant quenching, bacterial killing, and regulation of neutrophil apoptosis. Studies are ongoing to determine proline hydroxylation as a function of vitamin C concentrations in normal human fibroblasts. To date, these findings show that vitamin C function can be determined in relation to its concentration in living cells. Clinical goals of in situ kinetics are to characterize concentrations achieved in humans as a function of dose, mechanisms that control these concentrations, and functional consequences. To investigate dose concentration relationships, clinical studies were undertaken in healthy men and women inpatients hospitalized at the Clinical Center for 5-7 months. For the first time, the following were described: the relationship between vitamin C doses over a wide range and its concentrations in plasma and tissues; true bioavailability of vitamin C; vitamin urinary excretion in relation to dose; functional antioxidant consequences of vitamin C in vivo; a three component pharmacokinetics model of vitamin C distribution in humans; and potential adverse effects in relation to dose. Continuing analyses of the extensive data generated from these studies are ongoing. One striking finding from the clinical data was that orally ingested vitamin C over a very wide dose range resulted in tightly controlled plasma and tissue concentrations. Tight control is mediated by intestinal absorption, tissue concentration, and renal filtration coupled to reabsorption. There are three implications of tight control of vitamin C. First, it mmay be permissive of paracrine function, secondary to local higher concentrations of secreted vitamin C. Second, tight control may occur because chronic higher concentrations might have adverse consequences. However, tight control is bypassed transiently by intravenous administration, with implications for novel potential use of intravenous ascorbic acid as a drug. Third, tight control may be lost if any of the mechanisms responsible are aberrant in disease, especially renal filtration and reabsorption. Clinical and basic studies to explore all these possibilities are ongoing. Based on the clinical data, RDAs for vitamin C in the U.S. and Canada were revised upward in 2000 by the National Academy of Sciences and were also increased in the following countries: Germany, Austria, Denmark, France, Japan, and China. Because the known health benefits from vitamin C are from foods containing the vitamin, we recommend that vitamin C intake is from at least 5 servings of fruits and vegetables daily.